We started taking them when they were 5. By that time, they could be counted on to say, "I need to pee," they could be counted on to actually do that (restroom duties) in an unusual circumstance, and they could and would accurately say, "my left foot is cold," etc.
As we neared the fishing spot, the oldest child would pass out the hand warmers, body warmers, and toasti-toes, and they would all be pre-warmed.